This invention relates generally to heating, ventilating and air conditioning (HVAC) systems and more particularly to two stage systems having reduced wiring between a room thermostat and the system control.
The use of staged HVAC equipment is ever increasing. A typical two stage system has a low and a high mode of heat transfer. For example, two stage gas furnaces have allowed the use of low and high combustion for many years. Two stage systems have advantages over the typical single stage system. Firstly, greater comfort is allowed. By allowing a lower heat transfer mode when the heat load is lower in the home, the temperature essentially will not swing beyond the desired set point in contrast to a single stage system. Furthermore, during the low transfer mode the system is more likely to run longer which helps to eliminate stagnant air conditions. Secondly, greater efficiency is obtained by using a low transfer mode. That is, less energy is consumed since the need for the high transfer mode only occurs for a small percentage of the time.
Technology has changed in recent years to more readily allow the use of two stage systems. For example, the expanding use of electronics has allowed two stage gas furnaces to be less costly. The creation of two stage compressors has brought about the two stage heat pump and air conditioning. However, each of these systems requires the use of a two stage room thermostat to accomplish the full benefit of the two stage heat transfer system. These two stage thermostats require the use of a second control wire to the heating/cooling equipment. This presents a major problem when the two stage system is being used to replace an existing single stage system. The procedure for adding another control wire to the room thermostat involves tearing open the walls and ceiling of the home. This is a major cost adder and inconvenience in the replacement market and can serve as a reason for not upgrading the existing equipment. This is particularly onerous since higher efficiency and greater comfort systems dominate the replacement market.
It is an object of the present invention to provide a method and apparatus to overcome the limitations of the prior art described above. Another object of the invention is the provision of a method and apparatus for wiring room thermostats to two stage HVAC system controls which are inexpensive and simple yet reliable and which are particularly suitable for the replacement market.
Briefly, in accordance with the invention, a single control wire, such as an existing control wire to a room thermostat, can be used to select both the high and low heat transfer modes of operation of a two stage HVAC system. According to the invention, the control to be used in a two stage system has a power supply from a transformer, such as a 24 VAC transformer. This voltage is full wave rectified to create DC voltages for the control""s microcontroller and relays. The microcontroller based control determines if an input is xe2x80x9cONxe2x80x9d or xe2x80x9cOFFxe2x80x9d by looking at the phase relationships of the control signal. Additional information is provided on the single control wire by placing a diode in series with the signal. The diode is added to create a distinct signal recognizable by the microcontroller. Thus, by transferring the descriptions from xe2x80x9cONxe2x80x9d to xe2x80x9cHigh Heat Transferxe2x80x9d and xe2x80x9cDiode in Seriesxe2x80x9d to xe2x80x9cLow Heat Transferxe2x80x9d all of the control information needed for the two stage control system can be sent to the control system.